Human 5-HT3 receptor

Abstract

Recombinant materials for preparation of a new form of human 5-HT3 receptor are provided. These materials permit the production of said receptor, display of said receptor on host cells, and compositions of diagnostic and therapeutic utility.

Description

ACKNOWLEDGEMENT OF GOVERNMENT SUPPORT

This invention was made with U.S. government support under Grant Number 5R29 NS 34702-02 awarded by the Department of Health and Human Services, National Institutes of Health, National Institute of Neurological Disorders and Stroke. The U.S. government has certain rights in the invention.

TECHNICAL FIELD

The invention relates to synaptic receptors that mediate transmission in the peripheral and central nervous systems. In particular, the invention concerns a new human 5-HT3 receptor that expands the repertoire of targets for pharmaceutical compounds.

BACKGROUND ART

Synaptic transmission in the peripheral and central nervous systems is regulated by a multiplicity of ligand-gated ion channels. This superfamily of receptors includes the γ-amino butyric acid (GABA) receptor, the glycine receptor, and the acetyl choline receptor, as well as receptors responsive to serotonin (5-HT). The GABA-, glycine-, and acetylcholine-gated receptors are formed from mixtures of homologous subunits in various combinations. Serotonin-gated receptors fall into at least three subclasses. 5-HT1 and 5-HT2 receptors are described in U.S. Pat. No. 5,155,218. The 5-HT1 and 5-HT2 receptors are characterized by seven transmembrane regions; however, the 5-HT3 receptors are ligand-gated ion channels which are related structurally to the GABA, glycine and nicotinic acetylcholine receptors. A number of subtypes of 5-HT1 and 5-HT2 receptors have been found, but only a single 5-HT3 receptor unit has so far been detected in mouse, human and guinea pig tissues (Miriq et al. Science (1991) 254:432-437; Belelli et al. Mol Pharmacol (1995) 48:1054-1062; Lankiewicz et al. Mol Pharmacol (1997) 53:202-212). This subunit, designated herein 5-HT3A, has been 25 expressed in oocytes and HEK-293 cells and the recombinantly produced form can mimic many of the properties of the 5-HT3 receptors as they occur in tissues. However, there are sufficient differences that it is suspected that additional 5-HT3 subtypes exist (Bonhaus et al. J Neurochem (1993) 62:1927-1932; Van Hooft et al. J Neurochem (1997) 69:1318-1321; Hussy et al. J Physiol (1994) 418:311-323).

The existence of a multiplicity of subtypes of such receptors is important. Agonists and antagonists can be found that are selective for one subtype or another, thus resulting in different physiological consequences and expanding the repertoire of pharmaceutical substances available for therapy. This enables a more nuanced treatment of various conditions affected by receptor stimulation.

The class of serotonin receptors with which the present invention is concerned, the 5-HT3 class, is evidently involved in the induction of nausea and in behavioral disorders. Apud, Neuropsychopharmacol (1993) 8:117-130. Thus, antagonists of the 5-HT3 receptor would be useful in ameliorating the side-effects of many cancer therapeutic drugs (Gralla et al. New England J Med (1991) 305:905-909). Antagonists to this receptor would also be useful in controlling behavioral disorders (Rodgers et al. Psychopharmacol (1995) 117:306-312; Costall et al. Brit J Pharmacol (1993) 90:89; Zoldan etal. Lancet (1993) 341:562-563).

The present invention provides an additional subtype of the 5-HT3 subclass which thus provides an additional tool for evaluating the selectivity of candidate antagonists and agonists of the serotonin receptor class as well as providing a useful reagent for screening libraries of compounds for desired physiological activities described above.

DISCLOSURE OF THE INVENTION

The invention provides recombinant materials for the production of a previously undescribed subtype of serotonin receptor, designated herein 5-HT3B. The availability of these recombinant materials makes possible high throughput screening of candidate antiemetics and antipsychotic compounds as well as providing a tool for enhancing the selectivity of such compounds in particular individuals and in response to specific conditions.

Thus, in one aspect, the invention is directed to recombinant materials for production of 5-HT3B receptor protein. Two polymorphic forms of the protein have been found associated with a single nucleotide change in the encoding sequence. The amino acid sequences of these forms and set forth in FIGS. 1A and 1B. Also useful are sufficient contiguous fragments of these proteins so as to have substantial functional identity with that of the amino acid sequences of FIG. 1A or 1B when displayed on a cell surface, or proteins with amino acid sequences sufficiently homologous to display the same functional characteristics.

In other aspects, the invention is directed to cells displaying the proteins of FIG. 1A or 1B in a functionally active form and to methods to identify agonists and antagonists of the associated receptor function in assay systems using such cells. The agonists and antagonists identified according to this screen are useful in treating nausea and psychological disorders. The invention is also directed to antibodies that recognize the 5-HT3B receptors, which are useful in purification of this protein, and to oligonucleotide constructs that modulate the production of the receptor in tissues.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A (SEQ. ID No:2) shows the amino acid sequence of the human Ser¹²⁹ 5-HT3B receptor.

FIG. 1B (SEQ. ID No:4) shows the amino acid sequence of the human Tyr¹²⁹ 5-HT3B receptor.

FIGS. 2A (SEQ. ID No:1) and 2B (SEQ. ID No:3) show the nucleotide sequences of cDNAs encoding the amino acid sequences of FIGS. 1A and 1B.

MODES OF CARRYING OUT THE INVENTION

The amino acid sequences set forth in FIGS. 1A and 1B herein characterize a newly discovered human 5-HT3 receptor subunit, the existence of which was previously unknown. The availability of this receptor provides an additional tool for development of pharmaceuticals that are useful in controlling nausea, controlling anxiety, and in regulating other behavior disorders. This receptor contains 441 amino acid residues which display 41% amino acid identity with the human 5-HT3A subunit using the algorithm embodied in the GeneWorks 2.4 software package (Intelligenetics) to determine identity. This level of identity is similar to that found among different subunit classes of other receptor families such as the a and P classes of the GABA-, glycine-, and acetyl choline-gated channels.

It is recognized that minor changes in amino acid sequence of receptor proteins may not alter their specificity for activating ligands or their downstream signaling sequelae. Thus, included within the invention are proteins (and recombinant materials for their preparation) which proteins exhibit sufficient homology to the amino acid sequences shown for the 5-HT3B receptor in FIGS. 1A and 1B to behave in substantially the same manner. Typically, such protein sequences will exhibit 90% amino acid identity, preferably 95% amino acid identity, more preferably 98% amino acid identity and most preferably 99% amino acid identity with the referent sequence. In addition, minor portions of the sequence may be deleted without appreciably affecting the biological function of the protein and the proteins and recombinant materials of the invention include fragments containing a sufficient portion of the amino acid sequence depicted so as to exhibit the same biological function.

In addition, particular fragments of the receptor or of the nucleotide sequence encoding it are useful per se. For example, the extracellular portion of the receptor contains epitopes useful in generating an immune response as described below for the production of antibodies. In addition, such fragments may be formulated so as to elicit a cellular immune response directed against cells displaying the receptors for treatment of conditions where overproduction of the receptor is undesirably affecting the physical or psychological condition of the subject. Formulations which are designed to target the cellular immune response are generally known in the art.

In addition, fragments of the nucleotide sequence set forth in FIGS. 2A and 2B are useful as probes for detecting the presence of the gene or detecting the expression of the gene encoding the 5-HT3B receptor by measuring the mRNA levels in cells or tissues.

The invention relates to recombinant materials that can be used to produce the 5-HT3B receptor in suitable host cells which can be cultured for suitable assays. The nucleotide sequences encoding the receptor shown in FIGS. 2A and 2B represent the naturally occurring cDNA. The recombinant materials of the invention, however, include the degenerate forms of the sequences shown in FIGS. 2A and 2B, as well as nucleotide sequences that hybridize under stringent conditions with either the nucleotide sequences shown in FIGS. 2A or 2B or with their degenerate forms, thus representing sequences that encode homologous proteins. By "stringent conditions" is meant that hybridization will occur only if the nucleotide sequence to be hybridized is 90% homologous, preferably 95% homologous, more preferably 98% homologous, and most preferably 99% homologous to those shown in FIGS. 2A or 2B or their degenerate forms.

As is understood in the art, the relevant nucleotide sequences encoding the 5-HT3B receptor protein may be retrieved using the methods described herein, or other methods known in the art using the sequence information provided, or may be synthesized using commercially available instrumentation, or may be prepared using a combination of these techniques. Once obtained, the encoding nucleotide sequence will be manipulated so as to effect its expression in desired recombinant host cells. While the receptor of the invention can be produced in prokaryotic cells, other microbes, such as yeast or even in plant cells or their protoplasts, it is most useful when produced in higher eukaryotic cells such as insect cells, for example Drosophila or Sf9 cells, or animal cells, for example CHO cells, COS cells, oocytes, or HEK-293 cells. HEK-293 cells are preferred. For expression in oocytes, the nucleotide sequence may be supplied in the form of cRNA.

Suitable control sequences for expression in each desired host are by now well known in the art and expression systems for a multiplicity of appropriate hosts are available commercially. Methods for introducing the nucleotide sequence encoding the 5-HT3B receptor into host cells of various descriptions are also well known in the art. Typically, the encoding nucleotide sequence is expressed from an extrachromosomal vector such as a plasmid. However, if desired, an entire expression system may be introduced into the host cell chromosome or the nucleotide sequence may be placed under control of expression control sequences already present in the chromosome, for example by homologous recombination.

The mature 5-HT3B protein can be produced intracellularly, displayed on the surface of cells, or secreted into the medium. If the protein itself is desired, independent of the cells, it can be prepared as a mature protein or as a fusion protein. Fusion proteins may include additional elements useful in purification or labeling. For example, a series of histidine residues has been used as a convenient moiety to assist in purification of the attached protein. Such fusion elements may also provide convenient means for other manipulations of the peptide, such as coupling to label.

Alternatively, the 5-HT3B receptor can be displayed on the cell surface. The amino acid sequences set forth in FIGS. 1A and 1B herein represent the entire 5-HT3B subunit including its signal sequence. For display of the receptor at the cell surface, an alternative signal sequence may be substituted. Thus a heterologous signal sequence may be ligated to the 5' end of the nucleotide sequence encoding the protein which provides the functional 5-HT3B receptor. Preferably, the signal sequence associated with the 5-HT3A subunit may be employed, and ligated to the 5-HT3B encoding sequence upstream of the codon for amino acid 16-26, preferably 18-20.

Cells expressing the receptor protein of the invention are useful in screening compounds that are agonists or antagonists of the receptor.

To screen for agonist activity, cells displaying the receptor are contacted with a compound to be tested and the presence or absence or amount of the functional response to activation of the receptor is observed. For example, the electrical properties of transfected cells, such as HEK-293 cells, are recorded in the whole cell voltage clamp configuration as described by Hamil et al. Pflueg Arch Eur J Physiol (1981) 391:85-100, under visual control using an inverted microscope as described by Lankiewicz et al. Mol Pharmacol (1997) 53:202-212. To screen for antagonists, cells displaying the receptor are incubated in the presence of both a known agonist, such as serotonin, and the candidate compound. The effect of the candidate compound on the functional response to the agonist is then observed. Compounds that diminish the functional response are identified as antagonists.

Antagonists can also be identified simply by their ability to bind the receptor, either testing such binding directly, or, more commonly, in competition with a labeled known agonist or antagonist. Of course, a variety of labels can be used in either format, including enzyme, fluorescent, and radioisotope labels. A multiplicity of assay formats to assess the binding of a candidate compound to a target protein is known. In one commonly used assay, membranes prepared from transfected HEK-293 cells are incubated with tritiated GR65630 and bound ligand is separated from free ligand by filtration as described by Lankiewicz et al. Mol Pharmacol (1997) supra. Antagonists to the receptor will be useful as pharmaceuticals in treating the nausea associated with chemotherapy, in treating high levels of anxiety, and in other psychological disorders such as aversion, psychotic disorders, depression, cognition disorders, and drug addiction. A review of conditions that involve 5-HT3 receptors is provided by Greenshaw Trends Pharmacol Sci (1993) 14:265-270.

Under certain circumstances, pharmaceutical compositions containing agonists will also be beneficial as analgesics as described by Alhaider et al. J Neurosci (1991) 11:1881-1888.

In addition to, for example, small molecule antagonists identified by the assays set forth above, alternative mechanisms to antagonize the receptor response may also be employed. Antibodies which are immunoreactive with the extracellular portions of the receptor are useful in this regard. Antibodies may be prepared, for example using the epitopes that are external to the cell surface, which can be identified using art-known methods, in generally understood techniques by eliciting an immune response in a suitable vertebrate subject such as a rabbit, rat or mouse. It may be necessary to couple the desired epitopes to an immunogenic carrier such as KLH or diphtheria toxoid in order to enhance the immune response, or the epitopes may be included in a larger protein, including the native 5-HT3B subunit itself Epitopes may also be present in tandem in a single amino acid sequence in order to enhance immunogenicity.

The polyclonal antisera directly obtained from immunization are useful, for example, in purifying the 5-HT3B receptor protein and in assaying samples for the presence of this receptor. Antibodies also include antiidiotypes. Antibodies which have undesirable cross-reactivities can be identified and removed from antisera using suitable affinity chromatographic techniques. For use as a pharmaceutical in antagonizing the receptor in situ, monoclonal antibodies are preferred. (Of course, monoclonal antibodies can be used for purification and assay as well.) Techniques for preparing monoclonal antibodies are well known and suitable immortalized cells producing antibodies of the correct specificity can be assessed by using the recombinant cells displaying the receptor, for example, or using the immunizing epitopes as antigens in suitable assays such as ELISA or RIA. By suitable design of screening assays, monoclonal antibodies which are specific for the 5-HT3B receptor and thus do not significantly cross-react with other, perhaps similar proteins, can be identified.

Antibodies may also conveniently be produced recombinantly by retrieving the genes encoding the monoclonal antibodies described above. Recombinant expression permits the production of single-chain antibodies as well as chimeric and humanized forms of antibodies specific to the 5-HT3B receptor protein. These antibodies may be used as antagonists in the manner set forth above.

As is understood in the art, the variable region of antibodies is responsible for their binding affinity, and thus, fragments of whole antibodies may be used, such as the Fab fragment, the Fab' fragment, or the F(ab')₂ fragment; these fragments can be prepared by proteolytic cleavage of whole antibodies or may be produced recombinantly. Recombinant production also permits production of the single-chain F_v form.

Fragments of antibodies are particularly useful in context where only binding to target is required. Thus, in addition to use in therapeutic compositions where their activity as antagonists is exploited, the antibodies or fragments may be labeled and used as a diagnostic tool to locate high concentrations of serotonin receptors in various tissues. Accordingly, individuals with higher or lower expression levels than those exhibited by normal control subjects can be identified. Individuals with high levels of expression as compared to control normal subjects are characterized by disorders such as high levels of anxiety, and other psychotic disorders such as a propensity for drug addiction. Persons with low levels of expression may exhibit other psychological disorders. Suitably radio-labeled antibody fragments may be used to measure these levels of expression, for example, in the brain.

Alternatively, the activity of the 5-HT3B receptor can be modulated by controlling its production and display in a subject. Various techniques for inhibiting transcription or translation of a nucleotide sequence are known. Such control may be effected through antisense constructs utilizing the information provided by the nucleotide sequences set forth in FIGS. 2A and 2B. Antisense oligonucleotides can be supplied directly or expression systems for generating such antisense sequences may be employed. Further, transcription may be interrupted by utilizing nucleotide sequences that form a triple helix with the double-stranded gene. Sequence-specific oligonucleotides designed to form triple helices have been described by Dervan et al. Science (1991) 251:1360-1365 and by Cooney et al. Science (1988) 241:456-460.

The progress of the effect of the treatment described above to modulate the production of 5-HT3B receptors may, if desired, be monitored using the labeled antibodies as a diagnostic tool as described above.

The availability of the nucleotide sequence encoding the 5-HT3B receptor also provides a material useful in gene therapy to correct conditions characterized by an inadequate production of this receptor.

Thus, the invention provides a newly discovered serotonin receptor that is significant in regulating the flow of signals through the nervous system and which represents a target for pharmaceuticals for modulation of this system and its metabolic consequences.

The availability of the nucleotide sequence encoding the 5-HT3B receptor provides a control against which mutations in the gene can be measured in various subjects. Thus, having available the sequence of the control "normal" gene permits detection and identification of mutations. Tracking the occurrence of such mutations and correlating them with the incidence of disorders related to defective or inadequately produced 5-HT3B receptors provides a basis for genetic diagnosis of individuals with a propensity for such disorders.

The following examples are intended to illustrate but not to limit the invention.

EXAMPLE 1

Isolation of Human 5-HT3B

A cDNA library from human fetal brain was amplified by anchored PCR methodology as described by Hanna et al. Genomics (1997) 43:384-386. The oligonucleotide primers were:

(1) 5'-CTGGAACTCCAGCATGTTTGATGA (SEQ. ID No:5)

(2) 5'-TGAAGGTCAGGCTGCAATTCTGGA (SEQ. ID No:6)

These primers were selected based on nucleotide sequences contained in GenBank Accession No. AC002290 which consists of 20 unordered nucleotide fragments from a PAC clone (pDJ105h16) that was derived from human chromosome 11 q23.1. This clone is not annotated, but was identified as encoding a serotonin receptor by the observation that it includes three nucleotide fragments that encode peptides of 38, 56 and 71 residues, each of which displays 50-60% similarity to regions of a query sequence representing the human 5-HT3A subunit (Belelli et al. Mol Pharmacol (1995) 48:1054-1062).

The amplification protocol was at 95°C C. for 45 sec., 60°C C. for 60 sec., 72°C C. for two min. for 40 cycles using the XL-PCR system (Perkin Elmer).

A number of cDNA fragments were obtained and the terminal sequences were used to design primers 3 and 4 for amplification of the entire sequence of the 5-HT3B subunit.

(3) 5'-ggccggaagcttGATCTGCCCAGGAATGTTGTCAAG (SEQ. ID No:7)

(4) 5'-ccggccctcgagctgcagtctagaTACCACGCCGCCCCACAGTGCCCAG (SEQ. ID No:8)

The resulting cDNA was sequenced, and the open reading frame which encodes the 5-HT3B subunit was cloned into pCDM8 (Invitrogen).

The sequence of the open reading frame for two polymorphic forms of human 5-HT3B is shown in FIGS. 2A and 2B and the deduced amino acid sequences are shown in FIGS. 1A and 1B. The subunit contains 441 amino acid residues with 41% amino acid identity with human 5-HT3A extending over the entire length of the mature subunits. The only difference between the two forms is in the identity of the nucleotide at position 452 which is C. in FIG. 2A and A in FIG. 2B. This results in a change in the encoded amino acid so that the protein of FIG. 1A contains a serine residue at position 129 and that in FIG. 1B contains a tyrosine at that position.

EXAMPLE 2

Determination of 5-HT3B Expression in Tissues

Using nucleotides 936-1392 as a probe, Northern analysis of human mRNA samples derived from various tissues shows expression of 5-HT3B in brain, testes and kidney. No mRNA transcript was detectable in samples of heart, placenta, lung, liver, muscle, pancreas, spleen, thymus, prostate, ovary, intestine, colon or leukocytes. In contrast, 5-HT3A MRNA was detected only in intestine and colon, thus demonstrating that the 5-HT3A and 5-HT3B subunit genes have distinctive expression patterns. However, Northern signals are fairly weak, and it is thus possible that expression occurs below the level of detection. It is thus possible that 5-HT3A and 5-HT3B subunits could assemble in tissues where both are expressed.

EXAMPLE 3

Transient Expression of Functional 5-HT3B Receptor

The nucleotide sequence encoding the 5-HT3B protein is inserted into the expression vector pCDM8 and transfected into HEK-293 cells. The transformed cells transiently express the 5-HT3B receptor at their surfaces and are used to screen candidate agonist and antagonist compounds.

In more detail, the HEK-293 cells are transfected with pCDM8 containing the 5-HT3B protein encoding DNA by a procedure adapted from Chen, C. et al. Mol Cell Biol (1987) 7:2745-2752. The cells are maintained in MEM supplemented with 10% FBS, pen/strep and glutamine in 5% CO₂, 37°C C.

The transformed colonies are then assayed for expression of the 5-HT3B receptor using the voltage clamp assay or ligand binding assay described hereinabove.

EXAMPLE 4

Stable Expression of 5-HT3B Receptor

The nucleotide sequence encoding the 5-HT3B protein of FIG. 1A or 1B is inserted into the expression vector pRC/CMV and transfected into HEK-29 cells as described by Lankiewicz et al. supra. The transfected cells express the 5-HT3B receptor at their surfaces and are used to screen candidate agonist and antagonist compounds.

In more detail, the HEK-293 cells are transfected with pRC/CMV by a procedure adapted from Chen, C. et al. supra. The cells are maintained in DMEM supplemented with 10% FBS, pen/strep and glutamine in 5% CO₂, 37°C C.

Exponentially growing cells are removed with trypsin-EDTA and plated at a density of 1.5×10⁶ cells per 10 cm plate and incubated overnight in 5% CO₂, 37°C C. Transformed cells are selected with G418. The transformed colonies are then assayed for expression of the 5-HT3B receptor by electrophysiological and radioligand binding assays as described above.

Claims

1. A dna molecule which comprises a recombinant expression system, which expression system comprises a nucleotide sequence encoding a protein having the activity of human 5-HT3B receptor, said protein being encoded by the nucleotide sequence set forth in FIG. 1B (SEQ. ID. NO: 1) or in FIG. 2B (SEQ. ID. NO: 3) or said protein having amino acid sequence at least 95% homologous to that encoded by SEQ. ID. NO: 1 or SEQ. ID. NO: 3, operably linked to control sequences for expression of said encoding nucleotide sequence.

2. The dna molecule of claim 1 wherein said control sequences are heterologous to said encoding nucleotide sequence.

3. Recombinant host cells modified to contain the dna molecule of claim 1.

4. A method to produce human 5-HT3B receptor which method comprises culturing the cells of claim 3 and

recovering the receptor from the culture.

5. A human 5-HT3B receptor prepared by the method of claim 4.

6. The dna molecule of claim 1, wherein said nucleotide sequence does not encode the signal sequence encoded by SEQ. ID. NO: 1, or SEQ. ID. NO: 3, but encodes a signal sequence heterologous thereto, whereby said signal sequence is operably linked to said protein.

7. The dna molecule of claim 1, wherein said encoding nucleotide sequence encodes said protein having human 5-HT3B receptor activity as a fusion protein.

8. The dna molecule of claim 1, wherein the encoded protein exhibits at least 98% amino acid identity to the protein encoded by SEQ. ID. NO: 1 or SEQ. ID. NO: 3.

9. The dna molecule of claim 1, wherein the encoded protein exhibits at least 99% amino acid identity to the protein encoded by SEQ. ID. NO. 1 or SEQ. ID. NO: 3.

10. The dna molecule of claim 1, wherein the encoded protein is identical to the protein encoded by SEQ. ID. NO: 1 or SEQ. ID. NO: 3.

11. A recombinant host cell modified to contain a protein having human 5-HT3B receptor activity displayed at its surface, wherein said protein has an amino acid sequence set forth in FIG. 1A (SEQ ID NO:2) or 1B (SEQ ID NO:4) or an amino acid sequence having 95% amino acid identity thereto.

12. The cell of claim 11 wherein said receptor has an amino acid sequence set forth in FIG. 1A (SEQ ID NO:2) or 2A (SEQ ID NO:4).

13. The cell of claim 11, wherein said protein has human 5-HT3B receptor activity and has 98% amino acid identity to the amino acid sequence set forth in SEQ. ID. NO: 2 or SEQ. ID. NO: 4.

14. The cell of claim 11, wherein said protein has human 5-HT3B receptor activity and has 99% amino acid identity to the amino acid sequence set forth in SEQ. ID. NO: 2 or SEQ. ID. NO: 4.